Thread turning is a lathe operation that creates precise helical profiles—external or internal—by synchronizing tool feed with spindle rotation. This guide starts with a practical, brand-agnostic “how to,” then shows how Scandinavian Tool Systems’ QuadCut® and TwinCut® make the job faster and more reliable. 

Plan the thread 

  1. Confirm the spec: profile (ISO/UN/UNJ/Whitworth/Trapezoidal/ACME/Round, pipe types like NPT/NPTF), size, tolerance class, direction (RH/LH), starts, length of engagement, and relief requirements. 
  1. Choose full vs partial profile: full-profile generates crest/root to spec; partial-profile offers pitch flexibility (you’ll set crest height with depth). 
  1. Decide external vs internal method: external threads have more rigidity; internal threads need special attention to tool overhang and chip evacuation. 

Prepare the setup 

  • Workholding & support: Minimize runout (≤0.01–0.02 mm TIR for precision fits). Support long parts with tailstock/steady to prevent pitch ripple. 
  • Relief & runout: Add a relief groove or program a clean pull-out to avoid flank damage near a shoulder. 
  • Coolant/chips: Flood or high-pressure coolant improves chip evacuation in deep or fine-pitch threads; air blast can help in aluminum. 

Program the cycle 

  • Use your control’s threading cycle (e.g., Fanuc G76 / G92, Siemens CYCLE97/THREAD, Okuma/Mazak equivalents). 
  • Entry/exit: Lead in 1–2 pitches before the start; exit with a pull-out (chamfer or short arc) into relief. 
  • Infeed strategy: Prefer alternating flank or (for 60° threads) ~29–30° flank infeed to reduce load and chatter; radial infeed is simplest but harsher on the tool for tough materials. Many CNCs offer alternating-flank as a canned option.  
  • Spring passes: 1–2 light passes at final depth improve pitch-diameter consistency, especially in stainless or long overhangs. 

Set starting parameters 

  • Speed (surface m/min): start conservative and adjust to toolmaker data and rigidity (e.g., alu high; stainless and superalloys low). 
  • Feed: feed/rev equals pitch (metric) or 1/TPI (Unified). 
  • Pass count: typically 6–14 for external single-start; internal threads often need more, lighter passes. Use decreasing-DOC strategy. (See pass tables/cutting data later under STS.) 

Gauge and finish 

  • Use Go/No-Go gauges for functional fit; use wires/optics/CMM when pitch diameter and flank angles are critical. 
  • Deburr carefully; maintain sealing faces on pipe threads. 

Troubleshooting quick table 

Symptom Likely cause Fix 
Chatter/wavy flanks Overhang, speed too high, radial infeed on tough material Shorten overhang, support part, lower speed, switch to alternating/flank infeed, add passes 
Burrs at crest Worn insert, too large final DOC New insert, add spring pass, reduce last DOC 
Poor fit PD off, thermal drift Adjust wear offsets, stabilize temp, add spring pass 
Chips pack in groove Weak chip control or coolant Threading chipbreaker, increase flow/air blast, smaller DOC 

How STS tools make this easier 

QuadCut® (external & internal threading) 

What it is: An upright, square threading insert mounted with a large center screw and supported by large seat surfaces. Why it helps: stronger insert and seat, very high indexing accuracy, and four cutting edges instead of three—ideal for the high axial loads in threading entry/exit.  

Coverage: Full-profile and partial-profile inserts for ISO Metric, UN/UNJ, Whitworth/BSP/BSPT/PG, Trapezoidal (DIN 103), ACME/STUB ACME, Round (DIN 405), NPT/NPTF, NPSM, API, and more (external and internal ranges).  

Helix/cassette system: External cassettes are 1.5° standard; internal standard cassettes are 0.7°. STS provides ±helix cassettes (e.g., “-98.5”) so the tool’s presentation matches the thread’s helix—important for Trapezoidal/ACME. 

Infeed guidance & data: STS documents radial, flank, alternating-flank, and modified-flank infeed, noting benefits and trade-offs; tables give recommended cutting speeds by grade/material and typical pass counts by pitch. Use these as baselines.  

New LATUMA coating: Higher hot-hardness and oxidation resistance (tested on CF8M-class stainless) to safely push speeds when setup allows.  

Representative holders: Standard shank, slim blade holders for tight shoulders, Swiss-type axial holders, and cassette-type bodies so one toolholder covers multiple pitch ranges by swapping cassettes. 

TwinCut® (compact internal threading) 

Where it fits: Internal threads when the bore is limited (Ø18 mm and up) or the pitch is relatively large. Two cutting edges with a sintered chipbreaker; very high indexing accuracy lets you swap edges without re-setting dimensions. Larger holders accept replaceable internal cassettes to reduce cost.  

Profiles available: Partial-profile 60°/55°, ISO Metric/UN/Whitworth, Trapezoidal DIN 103, Round DIN 405, ACME/STUB ACME, NPT and NPTF. The catalog explicitly warns to choose NPT vs NPTF correctly for sealing performance. 

Holders & cassettes: Small internal holders (e.g., QNR/QNL 16–20 shanks) and cassette-type bodies (25–40 shanks) with insert families 14N/18N; catalog tables show minimum bore diameters and f-dimension offsets for correct setup. 

Cutting data & passes: STS provides recommended m/min by material/grade and typical pass counts by pitch for full-profile ISO/UN/Whitworth internal threads; note that fewer passes may work depending on rigidity.  

Quick, STS-aligned setup recipes 

External ISO M20 × 2.5 (general steel) 

  • Tooling: QuadCut external holder (cassette-type, e.g., QER 25 body + QER 25-12 cassette, 1.5° std) with a full-profile ISO insert matched to 2.5 mm pitch.  
  • Programming: Alternating-flank infeed in G76-style cycle; add 1 spring pass. Use STS speed/pass tables as baseline, then adjust to stability.  

Internal 1/2–14 NPT in a tight Ø22 mm bore 

  • Tooling: TwinCut internal holder (e.g., QNR 0016K-14) + 14N 14NPT insert (verify NPT vs NPTF). Use cassette-type holder if you want flexibility (QNR 0025P-C25 + QNR 25-14 cassette). 
  • Note: Gauge with the correct L1 plug and keep a light finishing pass for sealing quality. 

Ready to cut cleaner threads with fewer passes?

Get a tailored setup for your machine, material, and pitch—complete with starting speeds, infeed strategy, and the exact QuadCut®/TwinCut® insert and holder.

  • Request your threading setup → We’ll specify insert, holder/cassette, helix angle, and pass plan.
  • Ask a tooling expert → Send your drawing (thread spec, bore/OD, material, machine/control).
  • Get a fast quote → Lead times and pricing on STS QuadCut® and TwinCut®.